This invention relates to scintillation cameras which are commonly called gamma cameras. The invention is particularly concerned with improving the uniformity and resolution of scintillation cameras.
In nuclear medicine, scintillation cameras are used to detect gamma ray photons emitted from a body in which a radioisotope has been infused. The photons are emitted in correspondence with the extent to which the isotope is absorbed by the tissue under examination. With proper processing, signals corresponding with the photons may be used to develop a point-by-point image, corresponding with the emission pattern, on a cathode ray oscilloscope. A common camera system in use today is based on the camera of Anger as disclosed in U.S. Pat. No. 3,011,057. The Anger camera comprises an array of photosensitive devices such as photomultiplier tubes, usually hexagonally arranged, having their input ends adjacent a light conducting plate or disc. Beneath the disc is a scintillation crystal which converts incoming gamma photons into light photons or scintillations. A collimator is interposed between the scintillator and the body so that photons emitted by the body will impinge perpendicularly on the planar scintillation crystal.
The scintillations are detected by the array of individual photomultiplier tubes which view overlapping areas of the crystal, and well-known electronic circuits are used to convert the outputs of the photomultiplier tubes into x and y coordinate signals which are used to control a cathode ray oscilloscope in such manner that each point source of light formed on the oscilloscope tubes correspond with a point at a similar location in the crystal or on the body. The output signals are also used to develop a z signal which turns on the oscilloscope tube in accordance with the computed coordinates. The z signal is developed only if the energy of the scintillation event falls within a predetermined energy window. A photographic film may be used as an integrator of the large number of light spots appearing on the screen of the oscilloscope. A substantial number of scintillation events is required to make up the final picture of radioactivity distribution in the body tissue.
One problem in existing scintillation camera systems is that when a source of radioactivity having uniform distribution is placed close to the crystal disc and a photograph is made of the oscilloscope, the photograph will show non-uniformity which is characterized by "hot spots" under each photomultiplier tube and cold spots between the tubes. In other words, a spot or scintillation event actually occurring between the photomultiplier tubes is sensed as being partially shifted under the tubes, causing a decrease in spot density between the tubes and an apparent increase in spot density under the tubes. This phenomenon can be mitigated by moving the photomultiplier tubes further from the disc, but this decreases the ability of the camera to resolve small details. Hence, if small details are to be resolved and if uniformity or correspondence between the generated and displayed image patterns is to be maintained, the normal electric signals that exist in the system must be modified or corrected.
One method of obtaining correction with non-electronic means is illustrated in U.S. Pat. No. 3,774,032 which is assigned to the assignee of the present invention. In this patent the distribution of light as perceived by the photomultiplier tubes is altered by placing masks between the crystal and photomultiplier tubes so that light from certain areas of the scintillator crystal cannot go directly to the photomultiplier tubes. This reduces the output of the tubes for scintillations occurring directly under them but it permits light from other areas, that is, from between the tubes to go directly to the tubes. The result is better resolution and uniformity in the image.
It has been proposed heretofore to achieve the results obtained in the cited patent by use of electronic correction means. Without electronic or other correction, scintillations occurring in areas between the tubes appear to be, by inherent geometric phenomena, nearer to the tubes. This is manifested by what is called nonuniformity of the displayed image. More particularly, the image derived from a uniformly distributed isotope source is more dense or concentrated immediately under and near the tubes than in between the tubes. Electronic correction is further based on recognition that if the input and output signals of the preamplifiers are linearly related, the disproportionality between brightness and distance remains, but if the output is modified so that low level signals corresponding with noise are eliminated and high level signals corresponding with the scintillation event occurring at or near the center of the tube are suppressed, more uniform distribution of the light spots on the display will be accomplished. It has been proposed and demonstrated in the prior art that if the output of the preamplifiers is properly biased, high level signals can be clipped or suppressed which, in effect, amounts to reducing the gain of the preamplifiers for high amplitude signals or signals above a predetermined amplitude. Thus, the plot of preamplifier input signal versus preamplifier output signal is linear for a first comparatively low level signal range and it has a break point in it after which gain is reduced for higher level input signals.
A system using the single break point concept has been made and tested and found to produce better results than were obtainable with linear amplification over the entire input signal range. However, uniformity and resolution were still not optimized for there was still some evidence of nonuniformity or concentration of light spots where they should have been uniformly distributed. In other words, there were still localized "hot" and "cold" spots which appeared randomly throughout the crystal, varying from system to system and depending upon the individual characteristics of the components of the system.